When a printer such as an ink-jet printer, the number of tones that can output is limited, is used, the number of tones of image data is reduced to that the printer can express by a quantization process by means of a printer driver on a host computer, and that image data is then transferred from the host computer to the printer.
The size of image data to be transferred increases and the time required for transferring image data from the host computer to the printer increases with increasing resolution of printer, resulting in a low print throughput. In such case, the following method may be used. That is, the printer driver sends only tone information of a density pattern using a density pattern method, and the printer converts the received tone information into dots. In this method, the data size can be smaller than that of binary data to be directly transferred from the host computer to the printer. For example, when the resolution of a printer is 600 dpi, and a unit density pattern is formed by collecting a total of four dots to be output from the printer, i.e., 2 vertical dots×2 horizontal dots, five tones can be expressed, as shown in FIG. 1. That is, when the printer driver executes a 5-valued quantization process for 300-dpi pixel information, and sends its tone information alone to the printer, it can make the printer output a pseudo continuous tone image.
When image data is transferred from the host computer to the printer by the aforementioned method, the aforementioned 5-valued quantization data is expressed by quantization codes each having a given bit length, and the quantization codes are packed to undergo data transfer. In terms of this packing process (since data transfer is done in units of 8 or 16 bits), the bit length of each quantization code is 2, 4, or 8 bits, and a 4-bit quantization code is used in case of the 5-valued quantization data. Therefore, since this quantization data has only tone information for five values with respect to 16 tones that 4 bits can express, it becomes information with very high redundancy.
Even such highly redundant information, which expresses five tones using 4 bits, can be used while the data transfer rate or the memory size of the printer has a large margin. However, as the printer requires higher resolution and higher speed, the data transfer rate and the data size that the printer can hold pose a problem. That is, when highly redundant information that expresses five tones using 4 bits is transferred to the printer, this results in very poor efficiency.
In order to combat this problem without changing the unit density pattern, when the number of tones is reduced from five values to four values, the quantization code can be expressed by 2 bits. However, a reduction of the number of tones leads to loss of tone information, production of false contours, an increase in granularity, and the like, thus deteriorating the image quality of an output image.